Various exhaust after-treatment devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines. In the case of compression-ignition or diesel engines, a great deal of effort continues to be expended to develop practical and efficient devices and methods to reduce emissions of largely carbonaceous particulates otherwise present in the engine's exhaust gas.
An after-treatment system for a modern diesel engine exhaust typically incorporates a diesel particulate filter (DPF) for collecting and disposing of the sooty particulate matter emitted by the diesel engine prior to the exhaust gas being discharged to the atmosphere. A typical DPF acts as a trap for removing the particulate matter from the exhaust stream. The DPF may also contain precious metals, such as platinum and/or palladium, which serve as catalysts to passively oxidize soot and hydrocarbons present in the exhaust stream. In many instances, the DPF may be regenerated or cleaned using superheated exhaust gas to burn off the collected particulate.
The particulate matter included in the engine exhaust gasses may include carbonaceous soot particulates that may be oxidized to produce gaseous carbon dioxide, as well as other non-combustible particulates (i.e., ash) that are not capable of being oxidized. The composition and morphology of exhaust gasses is largely a function of the fuel, engine type, engine design, engine operation and control methodology, environmental operating conditions and other factors. For example, engine lubricating oil that passes into the combustion chamber and is partially burned produces the majority of ash. As a further example, combustion in gasoline engines may produce submicron organic matter (OM), as well as sulfates and elemental silicon, iron, or zinc or sulfur. The elemental silicon, iron and zinc are non-combustible particulates and may comprise ash. As another example, combustion in diesel engines may also produce OM, sulfates and elemental silicon, iron, zinc or sulfur, as well as soot and ammonium.
While the pressure drop across the particulate filter may ordinarily be a good proxy for trapped soot mass concentration, in certain temperature ranges and at certain nitrogen dioxide levels in the exhaust flow, the pressure drop may become a less accurate predictor. These inaccuracies may be due to, for example, passive and nonhomogeneous burning of soot in the filter that may change the soot distribution in the filter (i.e., reducing the correlation between pressure drop over the filter and soot mass in the filter). For example, nonhomogeneous burning may cause cracks in the soot layer, reducing the resistance to flow. Such soot estimation inaccuracies may either result in a decrease in the filtering efficiency of the particulate filter, or may cause the filter to be actively regenerated at lower soot concentrations, which may decrease fuel efficiency.